1. Field of the Invention
The present invention relates to a sheet feeding device which feeds a sheet to an image forming portion, and an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a printer or a copying machine, is provided with a sheet feeding device including a sheet feeding cassette being a sheet accommodating portion which accommodates a plurality of sheets, and a feeding portion which feeds the sheets accommodated in the sheet feeding cassette while separating the sheets one by one. As one example, the feeding portion of the sheet feeding device includes a feeding roller which feeds a sheet, and a separation roller which abuts against the feeding roller and separates a sheet. Also, as the sheet feeding cassette, a sheet stacking plate, on which sheets are stacked, is vertically movably provided. The sheet stacking plate is biased by a spring, and the sheet is pressed against the feeding roller. Thus, a feeding force is generated. In this configuration, the sheet feeding and the sheet separation are performed through the single feeding roller.
In such a sheet feeding device, when feeding a sheet, a top sheet stacked on the sheet stacking plate is discharged by rotating the feeding roller in a state where the feeding roller is pressed against the top sheet. Then, the discharged top sheet passes through a separation nip formed by the feeding roller and the separation roller which abuts against the feeding roller. In this manner, the sheet is separated one by one. Also, the sheet separated one by one is discharged to a conveying path which is directed toward an image forming portion (see Japanese Patent Laid-Open No. 2009-7086).
However, in the conventional sheet feeding device which performs the sheet feeding and the sheet separation through the single feeding roller, the sheet discharged by the feeding roller is conveyed while being bent after a sheet leading edge collides against a separation member such as the separation roller. In this case, a resistance force is applied to a sheet in a direction opposite to a conveying direction.
For this reason, when conveying the sheet, a conveying force greater than a resistance force generated by the collision of the leading edge of the sheet and the bending of the sheet needs to be applied to the sheet, but the resistance force differs according to a type of the conveyed sheet. Generally, since a sheet having a large basis weight has a high stiffness, the sheet receives a large resistance force. Therefore, as compared with a sheet having a small basis weight, it is necessary to apply a large conveying force. Therefore, when a sheet having a large basis weight is discharged, it is necessary to increase an abutting force between the sheet and the roller in order to apply a large conveying force.
However, in a case where the abutting force between the sheet and the roller is set to be large, a double feeding easily occurs so that multiple sheets are fed at a time when conveying a sheet having a small basis weight, that is, a sheet having a small resistance force.
Therefore, the present invention has been made in consideration of this phenomenon, and is directed to provide a sheet feeding device and an image forming apparatus, which can stably feed sheets having a different basis weight.